1. Field of the Invention
The present invention relates to thermal packaging for natural convection cooled electronics.
2. Description of Prior Art
Power converters are necessary to convert readily available forms of power, such as power from an ac outlet or from a battery, to regulated power required by many electronic devices including laptop computers. Some of the power during the power conversion process is inevitably converted to thermal energy which must be dissipated at a high enough rate to prevent excessive heat build up and product failure. The packaging used to contain the power converter usually includes the means to diffuse the extraneous thermal energy, otherwise known as the dissipated power.
Thermal requirements have been established within the electronic industry world-wide by various agencies such as Canadian Standards Agency (CSA), Underwriter's Laboratories (UL) and Technischer Uberwachungs-Verein (TUV) which define tolerable thermal limits that allow safe product operation. These thermal limits are usually in the form of allowable temperature increases from the device maximum specified operating ambient air temperature as well as maximum temperature limits. One applicable and particularly stringent standard is UL 1950, sections 1.4.7, 1.4.8, and 5.1, which specifies, among other things, proper thermal test procedures and allowable temperature rises from ambient.
The demands of the computer market require that the power converter for a laptop computer be designed as small and lightweight as possible. The size and weight of the converter, however, depends not only on elements of the converter itself but also on the allowable dissipative power density of the packaging. It is desirable, therefore, to increase the allowable dissipated power density of the packaging as much as possible without increasing its weight or cost significantly. Ultimately, this allows the size of the power converter to decrease and to stay within acceptable industry thermal standards.
Natural convection using air as the heat transfer medium is presently the preferred method to dissipate thermal energy from laptop computers and/or power converters. Air in the enclosure exhibits very low thermal conductivity which tends to decrease the tolerable dissipated power density of the power converter. Forced convection methods (a fan for example) and alternative heat transfer media are undesirable in laptop computer power converters since they would increase the size, cost and weight above acceptable limits. Since the thermal dissipation is limited to natural convection, several methods have been developed to enhance the dissipated power density of natural convection cooled electronics.
The most common electronic packaging method used in the industry to date for a natural convection cooled devices is to design a highly efficient converter and place it in a volume large enough, normally calculated based on 250 milliwatts per cubic inch (mW/in.sup.3) using the conventional heat sinks in air method, to dissipate an adequate amount of heat to meet thermal limits. If the package size necessary to house the device using this technique was considered too large from a marketing standpoint then the thermal designer would shrink the unit by applying existing technology such as the addition of a fan or encapsulation of the unit. Both of these are expensive but effective solutions to the problem.
Encapsulation is the predominate method used to solve thermally related packaging problems by increasing tolerable dissipative power densities of natural convection cooled electronics wherein all components are encapsulated, or potted, with a material exhibiting a high thermal conductivity. Encapsulation is very effective and can increase the thermal dissipation densities to approximately 1500 mW/in.sup.3 or more. The encapsulation materials, however, are expensive and difficult to use. The prevailing cost is due to labor for handling and curing the potting materials. The potting materials also possess high densities which add unwanted weight. Several of these materials cause problems for electronic devices like transformers and electrolytic capacitors during the curing process because mechanical stress is incurred. Furthermore, encapsulation does not directly dispose of the heat externally but rather redistributes the heat from the heat generating components to surrounding internal circuitry. This approach brings all components in the device closer to an isothermal situation. The now solid internal ambient temperature of the entire power converter increases, affecting heat susceptible components and overall circuit reliability.